1. Field of the Invention
The present invention relates to a solid-state image pickup device, and in particular, to a solid-state image pickup device which incluses photosensitive cells formed in a laminated layer of a photoconductive material such as an amorphous semiconductor, and an imaging apparatus using such a device.
2. Description of the Prior Art
A solid-state image pickup device whose photosensitive cells formed with a photoconductive layer such as an amorphous semiconductor in a layer-built structure is advantageous in that it has a large numerical aperture with respect to incident light, and thus has a high sensitivity, so that the degree of freedom is designing the spectral sensitivity is great and a quite wide frequency range in which the device is sensitive is obtained. Moreover, blooming can be effectively suppressed. However, the afterimage phenomenon takes place in such a solid-state image pickup device, and is a drawback in using the device for taking pictures, especially in the form of motion pictures.
The afterimage phenomenon occurs at the rise time and fall time of the light incident on the photosensitive cells. In a still picture camera, for example, having an optical shutter and an image pickup device which develops a video signal at intervals of a field time period (1/60 second) at a signal rate of the standard television format, the level of the output video signal exponentially increases during a time period of about ten fields after the shutter opened to reach the stationary state in some cases. This is referred to as "afterimage rise-time" in the specification. After the shutter is closed, the output signal level exponentially decreases during a time period of about three fields to reach zero. This is called "afterimage fall-time" in the description.
These afterimages are caused by photocarrier traps existing in an amorphous semiconductor substance used as a photoconductive layer. The rise-time afterimage occurs when photocarriers generated by an incident light are captured by the traps so that the number of movable carriers to be measured in the form of a signal current is increased while such traps are filled according to the probability corresponding to the Boltzmann distribution. This event causes a delay time required for the output video signal to be set to the stationary state. Similarly, the fall-time afterimage is caused when photocarriers captured by such traps are decreasingly released according to the same probability.
A measurement of the spectral sensitivity characteristics of the rise-time afterimage has revealed the fact that the characteristics change depending on the rise time. This is because an incident light having a longer wavelength reaches the deeper position measured from the surface of the photoconductive layer to excite photocarriers, which arrive earlier at the pn junction disposed under the photoconductive layer for storing and transferring photocarriers.
Paying specific attention to the afterimage characteristics which have been regarded as a drawback of an image pickup device of the layer-built type, we have come to an idea of an image pickup device positively utilizing the characteristics for the color separation of the video signals. In accordance with this idea, a color camera system can be implemented which does not require the provision of a color separation optical filter to be disposed on the photosensitive cell array. This system is by no means inferior to a field-sequential image pickup system with respect to the operation for supplying video signals without causing the color irregularity between generated video signals, even for a moving object.